The goal of the proposed research program is to develop a simple, precise, and accurate method for the measurement of permeability in hollow fiber membranes. For an asymmetric hollow fiber the proposed method may be useful for the separate determination of the resistance to mass transfer in the "bulk" and in the "skin" of the membrane for high molecular weight solutes. The method described herein involves the unsteady-state diffusion of a radioactive solute through a sealed section of a hollow fiber membrane. In an experiment sealed membrane tubes will be immersed in a flowing solute-free bath and the amount of solute remaining in the sealed tubes will be measured after a predetermined time. The membrane permeability will then be determined by curve-fitting the experimental data to the results of a rigorous mathematical analysis of unsteady- state diffusion in a "long" composite cylinder.